Sudan has prevented the United Nations' top humanitarian official from visiting refugees from the troubled Darfur region, who have fled to Chad.
Jan Egeland is known for speaking his mind
The government did not give a plane carrying Jan Egeland permission to fly over Sudanese territory to reach Chad.
On Monday, he was not allowed to visit Darfur. He said the government did not want him to see the conflict's victims.
Some 2m people have fled their homes in Darfur. The government denies backing militias accused of atrocities.
Some 200, 000 people have crossed the border to Chad but Mr Egeland was unable to visit them and has cut short his tour of the region.
He said he had experienced "systematic obstruction" from the Sudanese authorities.
Millions still rely on food aid and emergency relief
"One of the biggest and most effective humanitarian operations on earth... is in Darfur," Mr Egeland said.
"In 2006 it is changing dramatically for the worse and I think that is the background for why I was blocked again this year from going."
On Monday, Sudanese foreign ministry spokesman Jamal Ibrahim said the government had asked Mr Egeland to delay his visit because it coincided with a holiday to mark the birthday of the Muslim Prophet Muhammad.
He said that in the light of the Danish cartoons row, it would not be sensitive or safe for a Norwegian such as Mr Egeland to visit.
The BBC's Jonah Fisher in Sudan says Mr Egeland is known for his willingness to speak his mind and has been a strong critic of the government's role in Darfur's violence.
The UN Mission in Sudan has lodged a formal protest against the government's refusal to allow Mr Egeland's flight to land.
The Sudanese government is trying to stop the UN from taking over the peacekeeping mission in Darfur from the under-funded African Union.
Ahead of his visit to Darfur, Mr Egeland said UN relief efforts were being undermined by poor security.
"We are being attacked and our humanitarian services disrupted all the time," he told AFP news agency.
In early 2004 he labelled Darfur the world's worst humanitarian crisis.
Millions of people had fled their homes as militia rampaged through villages, burning, killing and raping.
There are no official figures on how many died in what the US called a genocide, though estimates range from between 100,000 to 400,000.
Our correspondent says that almost two years on, the media spotlight has gone but the crisis is far from over.
Fearing militia attacks, 2m people live in overcrowded camps totally dependent on international agencies for food and water.
The violence has less intensity but has become more complex, our correspondent adds.
New rebel movements have formed and existing ones have split and fought each other.
Pro-government militia continue to attack villages and camps with, according to many observers, the backing of the Sudanese army.
The government denies backing the Janjaweed and blames the violence on the rebels who took up arms three years ago.
Rebel movements from eastern Chad now use Darfur's anarchy as their base, creating insecurity in border areas.